The present invention relates to a device for measuring the angular deviation of a flat plate, and particularly relates to a device for measuring the angular deviation of a flat plate such as a video disk and a digital audio disk from a reference position.
FIG. 2 shows a conventional device for measuring the angular deviation of a flat disk. A light source 2 for emitting a light beam upon the disk and a position detector 3 which receives the light beam reflected by the disk are provided in the device. The light beam from the light source 2 is perpendicularly made incident upon the disk 1 in a reference position shown by a full line in FIG. 2. The light beam reflected by the disk 1 in the reference position returns to the light source 2. When the disk is inclined out of the reference position, the reflected light beam from the disk reaches the position detector 3.
If the disk is inclined by an angle 0 out of the reference position, as shown by a one-dot chain line 1' in FIG. 2, the incidence position of the reflected light beam on the position detector 3 changes depending on the angle .theta.. The distance d between the incidence position of the reflected light beam on the position detector 3 and an optical axis of the light beam from the light source 2 is expressed as a function of a distance l between the disk 1 and a light-receiving surface of the position detector 3 and the angle .theta. as follows: EQU d=l. tan 2.theta..
It is obviously understood from the above equation that the angle .theta. can be determined if the distance l is fixed and the distance d is measured.
However, when the disk is not only inclined but also displaced along the optical axis of the emitted light beam as shown by a two-dot chain line 1" in FIG. 2, the distance l is changed so that the angle .theta. of the disk 1 cannot be accurately determined in accordance with the above equation. This is a drawback.